Adds a child SPB to this SPB. A child SPB is one that is owned by this bag,
and whose entries can be treated as sub-entries of this bag. For example, a
if a bag, representing a pallet, were to contain another SPB under the key
of "Crates", and that SPB contained one SPB for each crate (one of which was
keyed as "123-45", and that SPB had a string keyed as "SKU" and another keyed
as "Batch", then the following code would retrieve the SKU directly:

Adds a delegate to the SPB under a specified key. When this entry is retrieved
from the SPB, it will first be located by key, and then be evaluated by calling
it, and the value returned from the delegate invocation will be returned to the
entity calling into the SPB. Example:

Retrieves an enumerator that cycles through all of the entries in this SPB.
If the entry is not a leaf node, then it can have its enumerator invoked,
allowing that entry's child list to be walked, and so forth.